Contador had minute traces of clenbuterol in samples taken during his 2010 Tour victory and will face WADA and the International Cycling Union in a landmark case at CAS next month with his title and integrity at stake.

The Spanish rider claims contaminated beef that was bought from a butcher in his native country was responsible for his failed doping tests.

"I don't think we can generalize from what is happening in one specific country," Niggli said, but acknowledged that China also had issues with livestock being fed banned drugs to improve the quality of meat.

"It is an illusion to say Mexico is the only country, but definitely there is a big difference depending where you are coming from," the WADA lawyer said.

Dvorak said FIFA had "no indications" of contamination problems in Spain or other countries.

Contador and the soccer players in Mexico all had their urine tested by the WADA-accredited lab in Cologne, Germany.

While Contador's sample had 50 picograms of clenbuterol per milliliter, the Under-17 World Cup tests showed a "majority" tested between 50 and 300.

Dvorak said FIFA was alerted to potential contamination when three players gave samples of 300-1,300 picograms in the opening days of the three-week tournament, in tests conducted by the WADA lab in Los Angeles.

Mexican health official Mikel Arriola said authorities had begun a program of arresting farmers and shutting down slaughterhouses.

"We are going to continue these inspections in order to avoid poisoning the general population and doping (athletes)," Arriola said.